Adam 9A4QAV is mostly known as the manufacturer of the popular LNA4ALL, a low cost low noise amplifier which is often used together with the RTL-SDR to improve reception of weak signals. He also sells an ADS-B bandpass filter and an ADS-B antenna, the latter of which we reviewed in a previous post.

Now Adam has come out with two new RF bandpass filters which are for sale. RF filters are used to block unwanted interference from other strong signals which can cause trouble, especially with low cost receivers such as the RTL-SDR. 

The first new filter that he has developed is for FLARM (FLight Alarm System). FLARM broadcasts at 868 MHz and is a protocol similar to ADS-B. It is used by Gliders and some Helicopters for collision avoidance. It is possible to decode FLARM with an RTL-SDR which allows you to track gliders on a map, as discussed in one of our previous posts.

The second filter is for amateur radio astronomers who wish to detect the Hydrogen Line at 1420 MHz. Hydrogen molecules in space occasionally emit a photon at 1420 MHz. A single emission can’t be easily detected, but space and the galaxy is full of Hydrogen and the net result is an observable RF power spike at 1420 MHz. This can be detected with a high gain antenna, LNA, RF filter and radio like the RTL-SDR. The Hydrogen line can be used to measure things like the rotation and number of arms in our galaxy. Filters are very important for radio astronomy work as man made interference can easily drown out the relatively weak cosmic signals.

Adam sells all his fully assembled filters for 20 euros, plus 5 euros worldwide shipping.